User equipment for physical layer automatic repeat request

ABSTRACT

A user equipment for physical layer automatic repeat request is disclosed. The user equipment comprises a higher layer automatic repeat request (ARQ) mechanism, a physical layer transmitter, a physical layer receiver, an acknowledgment (ACK) transmitter and an adaptive modulation and control unit (AMC). A higher layer ARQ mechanism generates data for transmission. A physical layer transmitter receives the data for transmission from the higher layer ARQ mechanism, to format the received data into packets for transmission. A physical layer receiver receives and demodulates received packets and retransmission statistics. An ACK transmitter transmits a corresponding acknowledgment for a given packet at the physical layer receiver. An AMC unit adjusts a particular encoding/data modulation of each packet using collected retransmission statistics.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/084,043, filed Feb. 27, 2002 which is a continuation of U.S. patentapplication Ser. No. 09/939,410, filed Aug. 24, 2001, both of which areincorporated by reference herein as if fully set forth.

BACKGROUND

The present invention relates to wireless communication systems. Moreparticularly, it relates to a modification to such systems by employinga physical layer (PHY) automatic repeat request (ARQ) scheme.

Proposed broadband fixed wireless access (BFWA) communication systems,using either single carrier-frequency domain equalization (SC-FDE) ororthogonal frequency division multiplex (OFDM) plan on using a highspeed downlink packet access (HSDPA) application. This application willtransmit downlink packet data at high speeds. In BFWA, a building orgroup of buildings are connected, either wirelessly or wired, andoperate as a single subscriber site. The data demand for such a systemis quite high for the single site's multiple end users requiring largebandwidths.

The current proposed system employs a layer 2 automatic repeat request(ARQ) system. Data blocks unsuccessfully transmitted to the subscribersare buffered and retransmitted from layer 2. The data blocks stored inlayer 2 are typically large, are transmitted for high signal to noiseratio (SNR) reception, are received with a low block error rate (BLER),and are infrequently retransmitted. Additionally, layer 2 ARQ signalingis typically slow requiring large buffers and long retransmissionintervals.

Accordingly, it is desirable to have alternatives in addition to a layer2 ARQ system.

SUMMARY

A user equipment for physical layer automatic repeat request isdisclosed. The user equipment comprises a higher layer automatic repeatrequest (ARQ) mechanism, a physical layer transmitter, a physical layerreceiver, an acknowledgment (ACK) transmitter and an adaptive modulationand control unit (AMC). A higher layer ARQ mechanism generates data fortransmission. A physical layer transmitter receives the data fortransmission from the higher layer ARQ mechanism, to format the receiveddata into packets for transmission. A physical layer receiver receivesand demodulates received packets and retransmission statistics. An ACKtransmitter transmits a corresponding acknowledgment for a given packetat the physical layer receiver. An AMC unit adjusts a particularencoding/data modulation of each packet using collected retransmissionstatistics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are simplified block diagrams of downlink and uplinkphysical ARQs;

FIG. 2 is a flow chart for using retransmission statistics for adaptivemodulation and coding; and

FIG. 3 is block diagram showing a multi-channel stop and waitarchitecture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 a and 1 b respectively show a downlink physical ARQ 10 anduplink physical ARQ 20.

The downlink physical ARQ 10 comprises a base station 12 receivingpackets from the higher layer ARQ transmitter 14 a provided in network14. The packets from transmitter 14 a are applied to the physical layerARQ transmitter 12 a in base station 12. The ARQ transmitter 12 aencodes the data with a forward error correcting code (FEC), appendserror check sequences (ECSs), modulates the data as directed by theadaptive modulation and coding (AMC) controller 12 c, such as by usingbinary phase shift keying (BPSK), quadrature phase shift keying (QPSK)or m-ary quadrature amplitude modulation (i.e. 16-QAM or 64-QAM).Additionally, for orthogonal frequency division multiple access (OFDMA),the AMC controller 12 a may vary the subchannels used to carry thepacket data. The physical layer ARQ transmitter 12 a transmits packetsto the subscriber unit 16 through air interface 14 by way of switch,circulator or duplexor 12 d and antenna 13. The transmitter 12 a alsotemporarily stores the message for retransmission, if necessary, in abuffer memory incorporated in the transmitter 12 a.

Antenna 15 of subscriber unit 16 receives the packet. The packet isinput into physical layer ARQ receiver 16 a through switch, circulatoror duplexor 16 b. At the receiver 16 a, the packet is FEC decoded andchecked for errors using the ECS. The receiver 16 a then controlsacknowledgment transmitter 16 c to either acknowledge (ACK) receipt of apacket with an acceptable error rate or to request retransmission by,preferably, withholding an acknowledgment signal or transmitting anegative acknowledgment (NAK).

The ACK is sent by ACK transmitter 16 c to the base station 12 throughswitch 16 b and antenna 15. The ACK is sent via the air interface 14 toantenna 13 of base station 12. The received ACK is processed by anacknowledgment receiver 12 b in the base station. The ACK receiver 12 bdelivers the ACK/NAKs to the adaptive modulation and coding (AMC)controller 12 c and to the transmitter 12 a. The AMC controller 12 canalyzes the channel quality to the subscriber unit 16 using statisticsof the received ACKs and may vary the FEC encoding and modulationtechniques of subsequent transmissions of the message, as will bedescribed in more detail. If the subscriber unit 16 acknowledges receiptof the packet, receipt of this ACK at base station 12 causes theoriginal packet, which was temporarily stored in a buffer memory, to becleared in readiness for the next packet.

If no ACK is received or a NAK is received, the physical layertransmitter 12 a retransmits the original message or selectivelymodified version of the original message to subscriber 16. At thesubscriber unit 16, the retransmission is combined with the originaltransmission, if available. This technique facilitates receipt of acorrect message by use of data redundancy or selective repeat combining.The packets having an acceptable error rate are transferred to higherlayers 16 d for further processing. The acceptable received packets aredelivered to the higher layers 16 d in the same data order in which thedata was provided to transmitter 12 a in the base station (i.e.in-sequence delivery). The maximum number of retransmissions is limitedto an operator-defined integer value, such as in the range of 1 to 8.After the maximum number of retransmissions are attempted, the buffermemory is cleared for use by the next packet. Decoding an acknowledgmentusing small packets at the physical layer reduces transmission delaysand message handling time.

Since PHY ARQ occurs at the physical layer, the number of retransmissionoccurrences for a particular channel, retransmission statistics, is agood measure of that channel's quality. Using the retransmissionstatistics, the AMC controller 12 c may vary the modulation and codingschemes for that channel, as shown in FIG. 2. Additionally, theretransmission statistics can also be combined with other link qualitymeasurements, such as bit error rates (BERs) and block error rates(BLERs), by the AMC controller 12 c to gauge the channel quality anddetermine whether a change in the modulation and coding scheme isrequired.

To illustrate for SC-FDE, the retransmission occurrences for aparticular channel are measured to produce retransmission statistics,(60). A decision on whether to change the modulation scheme is madeusing the retransmission statistics, (62). If the retransmissions areexcessive, a more robust coding and modulation scheme is used, (64),usually at a reduced data transfer rate. The AMC controller 12 c mayincrease the spreading factor and use more codes to transfer the packetdata. Alternately or additionally, the AMC controller may switch from ahigh data throughput modulation scheme to a lower one, such as from64-QAM to 16-QAM or QPSK. If the rate of retransmissions is low, aswitch to a higher capacity modulation scheme is made, such as from QPSKto 16-ary QAM or 64-ary QAM, (66). The decision preferably uses both theretransmission rate and other link quality measurements signaled fromthe receiver, such as BER or BLER, (62). The decision limits arepreferably set by the system operator.

For OFDMA, the retransmission occurrences are used to monitor thechannel quality of each subchannel. If the retransmission rate orretransmission rate/link quality for a particular subchannel indicatespoor quality, that subchannel may be selectively nulled from the OFDMfrequency set, (64), in order to preclude use of such poor qualitysubchannels for some future period. If the retransmission rate orretransmission rate/link quality indicates high quality, a previouslynulled subchannels may be added back to the OFDM frequency set, (66).

Using the retransmission occurrences as a basis for AMC provides aflexibility to match the modulation and coding scheme to the averagechannel conditions for each user. Additionally, the retransmission rateis insensitive to measurement error and reporting delay from thesubscriber unit 16.

The uplink ARQ 20 is similar in nature to the downlink ARQ 10 and iscomprised of a subscriber unit 26 in which packets from a higher layerARQ transmitter 28 a of the higher layers 28 are transferred to physicallayer ARQ transmitter 26 a. The message is transmitted to the basestation antenna through switch 26 d, subscriber antenna 25 and airinterface 24. The AMC controller, likewise, may vary the modulation andcoding scheme using the retransmission statistics of a channel.

Physical layer ARQ receiver 22 a, similar to receiver 16 a of FIG. 1 a,determines if the message has an acceptable error rate requiringretransmission. The acknowledgment transmitter reports status tosubscriber unit 26, causing the transmitter 26 a to retransmit oralternatively to clear the original message temporarily stored attransmitter 26 a in readiness to receive the next message from thehigher layers 28. Successfully received packets are sent to the network24 for further processing.

Although not shown for purposes of simplicity, the system is preferablyused for a HSDPA application in a BFWA system, although otherimplementations may be used. The BFWA system may use frequency divisionduplex or time division duplex SC-FDE or OFDMA. In such a system, thebase station and all of the subscribers are in fixed locations. Thesystem may comprise a base station and a large number of subscriberunits. Each subscriber unit may serve multiple users within one buildingor several neighboring buildings, for example. These applicationstypically require a large bandwidth due to the large number of end usersat one subscriber unit site.

A PHY ARQ deployed in such a system is transparent to the higher layers,such as the medium access controllers (MACs). As a result, PHY ARQ canbe used in conjunction with higher layer ARQs, such as layer 2. In suchcases, the PHY ARQ reduces the retransmission overhead of the higherlayer ARQs.

FIG. 3 is an illustration of an N-channel stop and wait architecture fora PHYARQ 30. The Physical Layer ARQ transmit function 38 may be locatedat the base station, subscriber unit or both depending on whetherdownlink, uplink or both PHYARQs are used. Blocks 34 a of data arrivefrom the network. The network blocks are placed in a queue 34 fortransmission over the data channel 41 of the air interface 43. AnN-channel sequencer 36 sends data of the blocks sequentially to the Ntransmitters 40-1 to 40-n. Each transmitter 40-1 to 40-n is associatedwith a transmit sequence in the data channel 41. Each transmitter 40-1to 40-n FEC encodes and provides ECS for the block data to producepackets for AMC modulation and transmission in the data channel 41. TheFEC encoded/ECS data is stored in a buffer of the transmitter 40-1 to40-n for possible retransmission. Additionally, control information issent from the PHYARQ transmitter 38 to synchronize reception,demodulation and decoding at the receivers 46-1 to 46-n.

Each of the N receivers 46-1 to 46-n receives the packet in itsassociated timeslot. The received packet is sent to a respective hybridARQ decoder 50-1 to 50-n (50). The hybrid ARQ decoder 50 determines theerror rate, such as BER or BLER, for the received packet. If the packethas an acceptable error rate, it is released to the higher levels forfurther processing and an ACK is sent by the ACK transmitter 54. If theerror rate is unacceptable or no packet was received, no ACK is sent ora NAK is sent. Packets with unacceptable error rates are buffered at thedecoder 50 for potential combining with a retransmitted packet.

One approach for combining packets using turbo codes is as follows. If aturbo encoded packet is received with an unacceptable error rate, thepacket data is retransmitted to facilitate code combining. The packetcontaining the same data is encoded differently. To decode the packetdata, both packets are processed by the turbo decoder to recover theoriginal data. Since the second packet has a different encoding, itssoft symbols are mapped to different points in the decoding scheme.Using two packets with different encoding adds coding diversity andtransmission diversity to improve the overall BER. In another approach,the identical signal is transmitted. The two received packets arecombined using a maximum ratio combining of symbols. The combined signalis subsequently decoded.

The ACK for each receiver 46-1 to 46-n is sent in a fast feedbackchannel (FFC) 45. The fast feedback channel 45 is preferably a lowlatency channel. For a time division duplex system, the ACKs may be sentin idle periods between upstream and downstream transmissions. The FFC45 is preferably a low speed, high bandwidth CDMA channel overlayingother in-band transmissions. The FFC CDMA codes and modulations areselected to minimize interference to other in-band transmissions. Toincrease the capacity of such a FFC 45, multiple codes may be used.

The ACK receiver 56 detects the ACKs and indicates to the correspondingtransmitter 40-1 to 40-n whether the ACK was received. If the ACK wasnot received, the packet is retransmitted. The retransmitted packet mayhave a different modulation and coding scheme as directed by the AMCcontroller 12 c, 26 c. If the ACK is received, the transmitter 40-1 to40-n clears the previous packet from the buffer and accepts a subsequentpacket for transmission.

The number of transmitters and receivers N is based on various designconsiderations, such as the channel capacity and ACK response time. Forthe preferred system previously described, a 2-channel architecture ispreferably utilized, with even and odd transmitters and receivers.

The PHY ARQ technique of the preferred embodiment provides a 7 db gainin signal to noise ratio (SNR) as compared to a system using only higherlayer ARQ. This occurs by operating at higher block error rates (BLERs)(5-20% BLER) and using smaller block sizes for layer 1 than is practicalwith higher layer ARQ alone. The decreased SNR requirement allows for:increased capacity by switching to high order modulation employing anadaptive modulation and coding (AMC) technique; lower customer premiseequipment (CPE) costs by using lower grade RF (radio frequency)components with the PHY ARQ compensating for reduced implementationperformance; increased downlink range which extends the cell radius;reduced downlink power in the base station (BS) to minimize cell-cellinterference; and increased power amplifier (PA) back-off when employinga multi-carrier technique.

1. A user equipment, comprising: a higher layer automatic repeat request(ARQ) mechanism configured to generate data for transmission; a physicallayer transmitter configured to receive the data for transmission fromthe higher layer ARQ mechanism, to format the received data into packetsfor transmission, wherein the packets are transmitted using anorthogonal frequency division multiple access (OFDMA) air interface, thephysical layer transmitter configured to retransmit an original orselectively modified packet, if an acknowledgment for that packet hasnot been received within a predetermined period of time; a physicallayer receiver configured to receive and demodulate received packets andretransmission statistics; an ACK transmitter configured to transmit acorresponding acknowledgment for a given packet at the physical layerreceiver, wherein a mechanism configured to receive the correspondingacknowledgment for the given packet operates transparently with respectto the higher layer ARQ mechanism; and an adaptive modulation andcontrol (AMC) unit configured to adjust a particular encoding/datamodulation of each packet using collected retransmission statistics; andwherein subchannels from an OFDM frequency set are selectively nulledwherein the use of a poor quality subchannel is precluded for apredetermined period and adding a previously nulled subchannel back intothe OFDM frequency set where a retransmission rate or link qualityindicates a high quality for the previously nulled subchannel.
 2. The UEof claim 1, wherein the packets are transmitted using a single carrierhaving a frequency domain equalization (SC-FDE) air interface.
 3. The UEof claim 1, wherein the received data is received in data blocks fromthe higher layer ARQ mechanism.
 4. The UE of claim 1, wherein theparticular encoding/data modulation is forward error correction (FEC).5. The UE of claim 1, wherein a physical layer ARQ mechanism and thephysical layer transmitter operate transparently with respect to thehigher layer ARQ mechanism.
 6. The UE of claim 1, wherein on conditionthat the collected retransmission statistics indicate a low number ofretransmissions, a higher capacity encoding/data modulation scheme isselected as the particular encoding/data modulation and on conditionthat the collected retransmission statistics indicate a high number ofretransmissions, a lower capacity encoding/data modulation scheme isselected as the particular encoding/data modulation.